The Internet has recently been utilized for various communications. E-mail and Web as non-real-time communication were mainly used when the Internet started spreading. According to advances in Internet technology, however, real-time communication in sound and image systems, such as a television, a telephone, and a monitoring camera, is recently used a lot in the Internet.
Since Quality of Service (QoS) is not taken into consideration in the Internet, the Internet is not suitable for the real-time communication. Therefore, the Internet is demanded to have a high communication speed in order to improve a real-time characteristic of communication via the Internet. For such a demand, for example, Patent Literature 1 discloses a cryptosystem, such as a streaming encryption, to improve the communication speed while maintaining security.
The number of users and a transmission data capacity abruptly increase (for example, high definition in image communication), and thus, the increasing of the communication speed of the Internet does not necessarily improve the real-time characteristic. From such a background, the real-time communication employs not the Transmission Control Protocol (TCP) communication having a low communication speed but the User Datagram Protocol (UDP) communication having a high speed.
However, the UDP communication may cause a packet loss and a reverse arrival order of packets, hence requiring a particular technique for a common key cryptosystem. For example, in an AES CBC mode of Arcfour encryption and block encryption of streaming encryption which are often utilized in the common key cryptosystem of the SSL cryptographic communication (TCP communication), the packet loss and the reversed arrival order of packets may cause a synchronization gap, and may prevent the receiving side from performing proper decryption from the time the packet loss and the reversed arrival order of packets occur.